Heterodyne interferometer for displacement measurement with amplitude quadrature and noise suppression

Widefield heterodyne interferometry using a custom CMOS modulated light camera

“…This arrangement was chosen as it is a simple interference to model and set up experimentally. Equations (8) and (9) describe the phase of a uniform wave, φ p (x, y), and a spherical wave, φ s (x, y) respectively;…”

“…The modulated light camera can be used to output a widefield AC image using the interference pattern. Quadrature demodulation has been used before to output the phase information of a heterodyne interference pattern [7,8]. The measured signal is mixed with an in-phase signal, I in , and a quadrature signal, Q in .…”

“…Onodera and Ishii produced work that used phasemeters [5] to determine point phases from the output of a heterodyne interferometer, with Riza and Arain, and Yokoyama et al developing a system that is high in precision [1,6]. Much like the work in this paper, Vegamota et al and Teng and Lang used quadrature demodulation to produce phase information [7,8], however, this was again done using one measuring sensor. Widefield heterodyne detection systems developed have been able to detect a single point on an interference pattern with 2D images produced by scanning across a pattern [1] presented by Riza and Arain and a system which uses a CCD camera [10] presented by Cervantes et al Despite being able to measure at high precision, the equipment used to generate different frequency light is expensive and methods of alignment are difficult.…”

Dark fringe operation and noise reduction with optical subtraction approach and power allocation

“…In above equation, the amplitudes of the AC photocurrent can be determined by sampling the output voltages from the photoreceiver and then taking least square fit with the sampled data [16].…”

“…In this report, we demonstrate an optical subtraction approach (OSA) based on previous study [16] for the common noise rejection without differential amplifier. This approach in conjunction with an optical power allocation strategy keeps the DC background and the signal amplitude of photocurrent close to zero simultaneously.…”